Superconductors by critical temperature, family, and pressure

Elements: mercury, lead, and niobium at ambient pressure

At ambient pressure, University of Illinois course notes give mercury, Hg, a critical temperature of about 4.2 kelvin; lead, Pb, about 7.2 kelvin; and niobium, Nb, about 9.2 kelvin [1]. These are elemental superconductors, with niobium supplying the highest ambient-pressure transition among the elements in this comparison. The figures describe superconducting transitions under low-field conditions. They are not universal current or magnetic-field ratings.

Use these elemental values as the starting entries in a superconductor critical-temperature table, rather than as specifications for complete devices. University of Illinois notes explain that early elemental superconductors lost superconductivity under modest currents, while alloy development opened stronger-magnet applications [1]. Niobium and niobium-titanium are different materials. An alloy's properties should therefore have their own entry, even when its name contains a familiar superconducting element.

A15 compounds: niobium tin and niobium germanium

Niobium tin, Nb3Sn, and niobium germanium, Nb3Ge, belong to the A15 intermetallic family. For ambient-pressure measurements, the iScience review lists Nb3Sn near 18 kelvin [2], while Solid State Communications reports an onset of 23.2 kelvin in optimized Nb-Ge sputtered films [3]. The latter result is an onset in a prepared film, not an interchangeable operating temperature for every niobium-germanium specimen.

The comparison also separates a temperature milestone from a conductor choice. The iScience review describes Nb3Sn as an established magnet-wire material whose superconducting phase forms during heat treatment [2]. Its brittle phase and heat-treatment route remain important even though its transition lies above that of elemental niobium. A warmer transition alone does not determine whether another A15 compound provides better wire, higher current in field, or easier coil fabrication.

Magnesium diboride: a warmer binary compound

Magnesium diboride, MgB2, has an ambient-pressure transition around 39 kelvin, according to the iScience review [2]. It is a binary compound containing magnesium and boron, distinct from the A15 and copper-oxide families. Its transition exceeds those of the conventional niobium compounds listed here. The same review describes it as an important development in bulk intermetallic superconductivity and discusses its preparation as practical wire.

For a family-by-family list, MgB2 deserves a separate entry rather than being folded into the cuprates. Its grain-boundary and conductor-processing problems differ from those of a layered copper-oxide ceramic [2]. Its transition temperature also remains a boundary, not a recommended magnet temperature. The iScience review describes declining current capacity as the applied field grows and discusses doping and pinning as ways to improve its useful performance [2].

Iron-based materials: distinguish bulk from interfaces

The iScience review lists bulk iron-based materials reaching about 55 kelvin in the iron-arsenide oxypnictide family at ambient pressure [2]. That figure is a family benchmark, not the critical temperature of every iron-containing superconductor. The same iScience table lists the barium- and strontium-based iron-arsenide family around 38 kelvin [2]. Chemical composition, carrier doping, and crystal structure all matter when comparing these entries.

Keep bulk materials separate from thin films and interface-specific reports. A transition in a substrate-supported layer is evidence about that layer under its particular conditions, rather than a demonstrated property of an unconstrained bulk specimen. The iScience review distinguishes the iron-based families and their conductor-development routes [2]. A useful list records the actual compound and sample form before comparing its temperature with a bulk family maximum.

Cuprates: ambient pressure and pressure enhancement

For ambient-pressure cuprates, the iScience review gives YBCO, the yttrium member of REBCO, a representative transition near 92 kelvin and Bi-2223 near 108 kelvin [2]. These values vary with composition and oxygen content. The mercury-based cuprate Hg-1223 supplied a higher historical ambient-pressure benchmark: Nature reports a maximum transition near 133 kelvin, supported by magnetic and resistivity measurements [4]. These entries belong to the layered copper-oxide family.

Pressure changes the comparison. Physical Review B reports Hg-1223 reaching 164 kelvin under quasihydrostatic pressure in experiments extending to 45 gigapascals [5]. That is a high-pressure result, not the ordinary ambient-pressure value of the compound. The chemical family remains the same, but the measurement condition has changed. A temperature list that combines these results without the pressure labels would conceal the experiment's essential distinction.

PNAS also reports an ambient-pressure resistive onset up to 151 kelvin in pressure-quenched Hg-1223 [6]. Pressure was applied during treatment and released while the sample was cold. The retained state was metastable, with limited thermal and temporal stability [6]. The reported onset should appear as a qualified processing result alongside the historical ambient-pressure entry. It should not replace a zero-resistance temperature or imply an ordinary, room-stable conductor.

Hydrides: high temperatures at megabar pressure

Nature reports a superconducting transition of 203 kelvin in the sulfur hydride system, with magnetic measurements at 155 gigapascals [7]. The paper identifies H3S as the likely high-temperature phase formed from the hydrogen-sulfide starting material under pressure [7]. Zero resistance, magnetic-field suppression of the transition, and an isotope shift supported the interpretation. This is a megabar-pressure benchmark, not an ambient-pressure transition for an available sulfide conductor.

For lanthanum hydride, LaH10, Nature reports superconductivity around 250 kelvin at about 170 gigapascals [8]. Zero resistance, an isotope effect, and magnetic-field dependence supported that result [8]. These hydrides supply the warmest well-supported benchmark entries in this survey, but they require sustained extreme compression. A search for the highest-temperature superconductor therefore needs a pressure condition attached to the answer. The hydride temperature alone does not identify a material usable outside the pressure apparatus.

Nickelates: sample form and onset matter

At ambient pressure, Nature reports a resistive onset of 45 kelvin in epitaxial lanthanum-praseodymium bilayer nickelate films [9]. That Nature paper places the film's BKT-like transition near 9 kelvin and its measured Meissner response near 8 kelvin [9]. These are different experimental criteria within the same study. Listing the onset as if it were a bulk magnetic transition would overstate the result, and the substrate-supported film is a distinct sample form.

Under high pressure, Nature reports a maximum resistive onset of 96 kelvin in samarium-substituted bilayer nickelate single crystals [10]. In a related composition, that Nature study reports a 92 kelvin onset and zero resistance at 73 kelvin under 21.6 gigapascals, with a Meissner response near 60 kelvin under 20.6 gigapascals [10]. The crystals were grown at ambient pressure, but superconductivity was measured under compression. These entries show why a useful temperature survey preserves onset, zero resistance, magnetic evidence, and operating pressure together.